In the field of digital printing, a digital printer receives digital data from a computer and places colorant on a receiver to reproduce the image. A digital printer may use a variety of different technologies to transfer colorant to the page. Some common types of digital printers include inkjet, thermal dye transfer, thermal wax, electrophotographic, and silver halide printers.
Often when printing digital images, undesirable image artifacts may result when an excessive amount of colorant is placed in a small area on the page. These image artifacts degrade the image quality, and can result in an unacceptable print. In the case of an inkjet printer, some examples of these image artifacts include bleeding, cockling, banding, and noise. Bleeding is characterized by an undesirable mixing of colorants along a boundary between printed areas of different colorants. The mixing of the colorants results in poor edge sharpness, which degrades the image quality. Cockling is characterized by a warping or deformation of the receiver that can occur when printing excessive amounts of colorant. In severe cases, the receiver may warp to such an extent as to interfere with the mechanical motions of the printer, potentially causing damage to the printer. Banding refers to unexpected dark or light lines or streaks that appear running across the print, generally oriented along one of the axes of motion of the printer. Noise refers to undesired density or tonal variations that can give the print a grainy appearance, thus degrading the image quality. Although these artifacts are presented in the context of an inkjet printer, it is known to those skilled in the art that similar artifacts commonly exist with the other above mentioned printing technologies also.
In a digital printer, satisfactory density and color reproduction can generally be achieved without using the maximum possible amount of colorant. Therefore, using excessive colorant not only introduces the possibility of the above described image artifacts occurring, but is also a waste of colorant. This is disadvantageous, since the user will get fewer prints from a given quantity of colorant.
It has been recognized in the art that the use of excessive colorant when printing a digital image needs to be avoided. Generally, the amount of colorant needed to cause image artifacts (and therefore be considered excessive) is receiver, colorant, and printer technology dependent. Many techniques of reducing the colorant amount are known for binary printers in which a halftoning process is used (typically inside a software printer driver program) to convert input digital image data into “on” or “off” states at each pixel. In such printers, the input image to the halftoning process is a higher bit precision image, typically 8 bits (or 256 levels) per pixel, per color.
U.S. Pat. No. 4,930,018 to Chan et al teaches a method of reducing paper cockle and graininess of inkjet prints utilizing multiple inks with different dye loadings. In this method, a given grey level can be reproduced a variety of different ways, some of which will use more colorant than others. The different ways to reproduce a given grey level are rank ordered according to the total ink coverage, and a selection is made by iterating through the order until one is found that satisfies a specified maximum coverage limit.
U.S. Pat. No. 5,515,479 to Klassen teaches a method for reducing marking material (i.e., ink) coverage in a printing process by determining the ink coverage for each pixel in an image, determining if too much ink will be placed on the page in a given area, and reducing the amount of ink to an acceptable level by turning “off” a fraction of the pixels in the given area. The determination of which pixels to turn off is made by using a processing order through each area which tends to randomize the turn off effect. While this method successfully reduces the amount of ink placed on the page in a given area, it can introduce pattern noise into the image because of the processing order method of selecting which pixels to turn off. Also, the pixels that are turned off in each color separation are not correlated, which can lead to a grainy appearance to an image region that should appear otherwise uniform.
U.S. Pat. No. 5,563,985 to Klassen addresses the problem of pattern noise by selecting which pixels to turn off in response to a random number function. While this method successfully eliminates pattern noise that can be generated in a given area, it can introduce random noise into the image because the selection of which pixels to turn off is determined by a random process. While this may be visually less objectionable than pattern noise, it is still not optimal.
U.S. Patent No. 5,012,257 to Lowe et al. describes a “superpixel” printing strategy to reduce bleed across color field boundaries. This strategy limits printing to no more than two drops of ink per cell or pixel, and no more than a total of three drops per superpixel, where a superpixel consists of a 2×2 array of pixel cells. This strategy controls bleed, but at a penalty in terms of color and spatial resolution.
U.S. Pat. No. 6,081,340 to Klassen teaches a method for reducing marking material (i.e., ink) coverage in a printer that has a nonlinear marking material coverage. As understood, this method applies to a halftoned image signal where the number of gray levels in the image has been reduced to match the number of available printing levels in the printer. A coverage calculator is then used to determine the amount of marking material that is present in a local 8×8 region of the current pixel. This method is disadvantaged because it operates after the halftoning process, and is therefore required to sample a region of the halftoned image data in order to estimate the marking material coverage, which can be time consuming. Also, the process of reducing the marking material coverage is limited to turning off integer numbers of discrete dots, therefore limiting the fidelity of the reduction step.
The above mentioned references teach methods of reducing artifacts associated with excessive colorant usage by utilizing methods that operate on the digital data after halftoning. That is, the above techniques operate primarily on bitmaps of image data where each pixel is represented by a code value of 0 (“off”, meaning no colorant), or 1 (“on”, meaning fall colorant). At this point in the imaging chain of a digital printer, much information has been lost due to the halftoning process, and accurately controlling the total colorant amount becomes more costly and less accurate relative to a pre-halftoning algorithm. U.S. Pat. No. 5,633,662 to Allen et al. teaches a method of reducing colorant using a pre-halftoning algorithm that operates on higher bit precision data (typically 256 levels, or 8 bits per pixel, per color). However, this method is intended for a binary printer where the halftone dot area is substantially linear with digital code value, and therefore the amount of colorant placed on the page is substantially linear with the digital code values that are used to drive the printer. In general, this arrangement will not be optimal for a multilevel printer.
In a multilevel printer, the colorant amount is typically not linear with digital code value. That is, if the digital code value (in a pre-halftone algorithm) is reduced by a certain percentage, the colorant amount is typically not reduced by the same percentage. In fact, the percentage of colorant amount reduction will typically vary based on the density (lightness/darkness) of the pixel. A printer with this characteristic is not handled well by any of the prior art methods, as they all assume that colorant amount is linear with digital code value.
Thus, there is a need for a colorant reduction algorithm which can be applied to a multilevel printer to provide for high quality images free from the artifacts associated with using excess colorant.